1. Field of the Invention
The present invention relates to a method of making axial alignment of a charged particle beam with at least three stages of multipole elements. The invention also relates to a charged particle beam system capable of making this axial alignment.
2. Description of Related Art
In a charged particle beam instrument (such as a transmission electron microscope (TEM) or scanning transmission electron microscope (STEM)), aberration correction is an essential technique for obtaining high spatial resolution. Especially, positive spherical aberration produced by an objective lens is a typical factor to limit spatial resolution (see H. Sawada et al., Journal of Electron Microscopy, vol. 58 (2009), pp. 341-347).
Today, it is widely known that this positive spherical aberration can be corrected by the use of negative spherical aberration produced by hexapole elements or the like. A hexapole element produces a three-fold astigmatism that is a second-order aberration. In spherical aberration correctors, hexapole elements of this construction are arranged in two stages such that their three-fold astigmatisms cancel out each other (see H. Rose, Optik, Vol. 85 (1990), pp. 19-24 and H. Haider et al., Nature, vol. 392 (1998), pp. 768-769).
The aforementioned aberration correction technique is fundamentally correction of third-order spherical aberration. Corrections up to the fourth-order aberration can be made by axial alignment, which can be achieved using an optical system. A fifth-order spherical aberration can be corrected if the distance between the objective lens and the aberration corrector is controlled optically. However, corrections up to still higher orders of aberrations cannot be achieved completely. On the other hand, correction of astigmatism of the same order (fifth order) (i.e., six-fold astigmatism) is not yet achieved. Therefore, in a spherical aberration corrector consisting of two stages of hexapole elements, six-fold astigmatism is a major aberration which remains after correction of third-order spherical aberration and which is a factor limiting spatial resolution. Where six-fold astigmatism cannot be corrected, further improvement of spatial resolution cannot be anticipated.
Therefore, in the aberration corrector of JP-A-2009-054565, three stages of three-fold fields are used to correct six-fold astigmatism. In this aberration corrector, three-fold symmetric fields produced by the middle and rear stages of multipole elements are distributed in certain rotational relationships to the three-fold symmetric field produced by the front stage of multipole element within the plane perpendicular to the optical axis.
When axial alignment of an electron beam with multipole elements is made, it is customary to deflect the beam using a deflector or the like. In the case of an imaging system aberration corrector, deflection for the axial alignment is done while grasping residual aberration in the aberration corrector having multipole elements by tilting the beam relative to an amorphous sample, taking an image of the sample, and subjecting the image to Fourier transformation. In the case of this illumination system aberration corrector, the processing is continued until a geometrical figure of a desired shape is obtained while monitoring variations in the distortion of the geometrical figure, known as a Ronchigram, which is observed on a diffraction plane when the electron beam is focused onto the sample. In the case of a two-stage multipole element system, the source of appearing aberration has been identified and so it is relatively easy to make axial alignment of the beam with both multipole elements.
In the case of axial alignment with three or more stages of multipole elements, if the axial alignment with the first stage of multipole element produces a new aberration, it is very difficult to identify which of the middle and rear stages of multipole elements has brought about the aberration. If a further stage of multipole element is added, the difficulty increases vastly. Accordingly, a quite long time would be consumed in making the axial alignment.